1. Field of the Invention
The present invention relates generally to lawn sprinkler systems and, more particularly, to lawn sprinkler systems which can be automatically controlled to vary the amount of water applied to the lawn.
2. Description of Related Art
The growth and maintenance of many plants, trees, and crops is often dependent upon an adequate or regular supply of water. Indoor plants can be watered by hand at predetermined intervals. Similarly, many crops are automatically watered with sprinkler systems at regular intervals.
Most lawns or cultivated fields are too large to water all at once with an automatic watering system. The flow capacity and associated pressure of a typical watering system is inadequate to accomplish a single watering of the entire area. Automatic lawn sprinkler systems are thus typically divided into sections, zones, or stations, with each station being automatically watered consecutively during a given watering period. A typical watering period for a lawn, for example, might range between several times a day to once in several weeks.
In addition to the low-pressure problems associated with many municipal watering distribution systems, water shortages are often encountered, as well. Many municipalities have imposed watering restrictions, based upon either the municipal water distribution system's inadequate ability to keep up with demand, water shortages, or a combination of both. A typical watering restriction imposes a maximum number of permissible waterings per week, for example. Although watering can be scheduled at any time during a given day, it is generally desirably to water in the early morning hours when there is less wind and consequently less evaporation.
A typical automatic lawn sprinkler system can be programmed to deliver a predetermined amount of water at each pre-set watering period, per watering zone. Many lawn sprinkler system controllers can be easily adjusted by manual programming in order to change the watering time by a desired percentage. Even with this manual adjustment capability, optimal watering of lawns, for example, can be difficult to achieve. During periods of significant rainfall, a lawn sprinkler system can over-water a lawn, potentially damaging the lawn and wasting water which could otherwise be conserved. Even if a user attempts to manually adjust for the rainfall, an inconsistent manual control of the lawn sprinkler system can damage a lawn by applying too much or too little water. An inexpensive, reliable, and accurate device which could automatically detect a moisture level of soil, and apply water accordingly, would be advantageous.
Various devices have been implemented in the past for sensing moisture. U.S. Pat. No. 4,737,629 to Iwama et al. discloses an example of a control system, which incorporates a light emitter and a light receiver in order to detect the deposition of water drops on a wind screen. The control system detects the deposition of water drops on the wind screen by measuring a degree of optical scattering, which is a function of the amount of water drops on the wind screen. A control system automatically activates a defogging apparatus when a specified amount of water is detected on the wind screen.
Although the prior devices have incorporated moisture detecting systems to achieve various ends, such as defogging a wind screen, these devices have not optimally incorporated an economical, accurate, and efficient moisture sensor for the purpose of automatically watering a lawn, based upon an amount of moisture detected in the soil. Soil moisture sensors have existed in the prior art, but these devices have been prohibitively expensive, complex, inaccurate, unreliable, and/or susceptible to changes in soil salinity. Examples of existing soil moisture sensors (not necessarily prior art) include tensiometer, gypsum block, fiberglass block, conducting electrodes, dielectric constant of a capacitor, materials that change size in moisture, lysimeter, hygrometers, psychrometer, time domain reflectometer, neutron absorption, X-ray computed tomography, nuclear magnetic resonance, and microwave attenuation. U.S. Pat. No. 5,355,122 to Erickson discloses an apparatus, which can disable a sprinkler system upon the detected occurrence of a rain shower, but soil moisture is not sensed. The device utilizes a receptacle for accumulating rainfall, and disables the sprinkler system when a predetermined amount of rainfall accumulation is obtained within the receptacle. Thus, the watering system follows a predetermined watering schedule, unless a significant rain shower is detected. This prior art apparatus is not effective for controlling a moisture content of soil within a potted plant, for example, which is not watered by sprinklers or rainfall. Additionally, since this prior art device does not measure soil moisture, for example, the device does not account for differences in humidity and light exposure, both of which may contribute to different soil-moisture change rates. Additionally, the open receptacle of the device may become covered with debris, such as insects, leaves, or earth born contaminants. The prior art apparatus, additionally, is not very accurate, since the apparatus basically performs a binary determination of whether or not a next scheduled sprinkler watering should be executed.